\begin{tikzpicture}[->,>=stealth',shorten >=1pt,auto,node distance=5em,semithick]
    \tikzstyle{every state}=[text=black]
    \tikzset{nearnodes/.style={node distance=2em}}

    \node[state] (1) {1};
    \node[state] (3) [right of=1] {3};
    \node[state] (6) [below of=3] {6};
    \node[state] (5) [right of=3] {5};
    \node[state] (4) [right of=6] {4};
    \node[state] (end) [right of=5] {};

    \node[nearnodes] (+1) [left of=1] {+};
    \node[nearnodes] (-end) [below right of=end] {-};
    \node[nearnodes] (-4) [below right of=4] {-};
    \node[nearnodes] (-5) [below right of=5] {-};

    \path (1) edge node {$a$} (3); 
    \path (1) edge node {$b$} (6);
    \draw (3) edge [dashed] (5);
    \draw (6) edge [dashed] (5);
    \draw (6) edge [dashed] (4);
    \path (4) edge node [below] {\ooalign{$\bigvee$\cr$-$}} (end);
    \path (5) edge node  {\ooalign{$\bigvee$\cr$-$}} (end);
    
\end{tikzpicture}